Box barge hull shape - which is better?

Discussion in 'Hydrodynamics and Aerodynamics' started by Justaguy, Jan 30, 2016.

  1. waikikin
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    waikikin Senior Member

    Justa,
    There's nothing ironic about it, you are using a forum to find information, apparently very specific but without including other important supporting information. The nature on forums is that many can contribute, some you might find useful... or not, it should be clear by now that without the other info your question is aimless- example : what is the beam of your barge 1/2" or 10' or 20'.. this and all other parameters make a difference.
    I think you need professional help, once you pay for information you might value it more greatly, in the meantime you have been disrespectful of worthy input, that's probably why your points have been ripped.
    Jeff.
     
  2. MoePorter
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    MoePorter Junior Member

    Justaguy...stop trolling & let the 100+ years of experience trying to help you...help you. Really. It's an interesting topic, let it happen. Moe
     
  3. Justaguy
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    Justaguy Junior Member

    That's exactly the problem -- a persistent few of them are not actually trying to help. They began by preening, and are now engaged in group ego defense.

    Go back and read my first post, and any other post where I tried hard to get good answers and keep this on topic, and then tell me again that I'm trolling. Yeah, there are a couple of posts where I let them get under my skin and it showed. No one's perfect.

    Long before I ever posted, I read this forum for quite a while. I've seen a lot of good discussions and advice given. Unfortunately, I've also seen a repeated, unhelpful reaction from a few ... those belonging to the "meager information" and "crystal ball" crowd. I've watched them belittle people who sincerely came here for help, but for one reason or another, didn't use the right buzz words. I will not tolerate it.

    Longevity is not impressive. Mutually respectful, on-topic replies are. My question could have been answered easily by any person who actually wanted to answer it provided they are competent in the field, as has been demonstrated by a couple contributors. Even though I'm the one who asked this question in the beginning, I can come close to answering it myself, at least in template form:

    "The pros and cons of Bow Shape 1 are __, __, __ and ___, ___, ___. The pros and cons of Bow Shape 2 are __, ___, ___, and ___, ___, ___. All other things being equal, Bow Shape __ is more hydrodynamically efficient because __________."

    How is it possible that "100+ years of experience" is incapable of a) seeing the appropriateness of such a paragraph to my question, b) writing that paragraph and c) competently filling in the blanks, when I, a novice, can get part of the way there on my own?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Still doing angry ant impersonations I see. You gave everyone their chance to help you, and they blew it !
     
  5. Justaguy
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    Justaguy Junior Member

    Still have nothing substantial to add I see.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    You have to give something substantive, such as what size vessel is proposed, load to be carried , calm or semi-sheltered water, type of propulsion etc. Nothing so far, except a fixation on a bow profile, questions about which must be answered in the spaces provided, or else !
     
  7. JRD
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    JRD Senior Member

    I think Tansl gave the only possible absolute answer early on by suggesting that water doesn't like to go around sharp corners. Devoid of any other detail that's as good an answer as you will get. There is no one magic formula to say by how much or under what circumstances it will be better.
     
  8. Justaguy
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    Justaguy Junior Member

    Thanks for the clarification.
     
  9. GTO
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    GTO Senior Member

    I think Justaguy, that what you have failed to realize or accept is that a boat's hull must be considered in its entirety, at least all of it at and below the waterline.
    Any protuberance into the water from the hull, has its characteristics modified by the hull to which it is attached.
    That is why there really isn't a simple answer to your question.

    Given a set of water conditions, speed of the hull through the water, and other things I have no clue about, a given bow shape may or may not be acceptable.
    Having read the thread, no one here actually started off trying to be unhelpful.
    You in your ignorance of boat design, just didn't realize it. Note I said ignorance, not stupidity. You just don't know what you don't know.

    A boat hull is a complete 3D structure. Each component affects the performance and characteristics of every other component.
    In your particular case/question, any bow unattached to any hull is a perfect bow, since it isn't required to do anything.

    Your question would best be answered in an experimental way, that all depends upon the setup in the experiment.
    For example, is the back of the bow open to water or is there a bulkhead there that would affect the turbulence generated by the free standing bow?
    So many parameters to consider. Too many for a forum probably.

    No circling of the battlewagons here, just trying to help you understand that people here are trying to help you.
     
  10. Justaguy
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    Justaguy Junior Member

    I'd like to think that about all participants. Thanks for the thoughtful reply, GTO. I'll give it some thought.

    I've led with statements about what I don't know, so I agree with you there. However, I'm still struggling with the idea that, in a forum on technical topics, concept isolation for the purpose of explanation seems to be an unfamiliar method. In education, we isolate concepts all the time to teach them. In any mechanical troubleshooting, isolation is the primary technique. But here, somehow, isolation not only isn't applicable, but supposedly impossible. That's very puzzling.

    Justaguy
     
  11. Barry
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    Barry Senior Member

    In your first post, you wanted the contributors to make a decision on 3 shapes that you provided.
    For a BARGE, which most people assumed, as did I, that this would be a large flat bottom boat used to carry heavy items at slow speeds.

    So to the question of the shape. Even though your rounded bilge is the best of the three profiles illustrated for a 4 -5 knot barge, it is not the best for least drag. You can merely Google Coefficients of Drag for Shapes to find this out. Note that this is empirical data. Ie data produced from experiment. Hang a shape into a stream and you can determine the drag coefficient.

    Then you asked WHY after also stating that "you don't know much about naval architecture ( but am learning)

    The WHY of the best shape is not learned with a few posts, though one contributor simplified an answer. The WHY is learned with courses in hydrodynamics which requires at least three or more University math courses to begin to understand hydrodynamics.

    And this is only one part of a naval architects training.

    You demanded and pushed back when many learned design people kept asking for more information knowing that you cannot easily simplify many years of education and experience.

    An extremely simplified explanation for the best of the three profiles would be
    1) at the front, the water is accelerated more smoothly with the curve than with say the 45 degree angle. If the acceleration is steady rather than abrupt the resistance will be a little less with the curve than the incline
    2) at the back, the curve will allow some pressure recovery due to its smoother shape rather than an abrupt transition, which can produce more drag

    Then you stated that you are not really thinking of a barge to carry heavy things at slow speeds but are wanting 7 -8 knots and might power it with engines or even sails
    Without the parameters that everyone is trying to get out of you, a 7 -8 knot barge, in my mind anyway is not what your original post suggested.
    As you have not given us length, depth, width, the suggestion that it might be powered by sail introduces another issue and that would be changes in drag due to the heeling effect that a sail produces.The narrower the hull, generally, the more heel. So when you threw this in, parameters are changing

    Your comment "if you are going to teach someone something, you have to teach meet them where they are at, not where you are at" is not really relevant with your inquiry.
    The WHY one profile is better than another from the Theoretical view requires advanced math and a basic understanding of dynamics.

    So if you are trying to actually get to an answer that can be provide by the competent contributors, then come to the table with a more complete set of parameters.

    Your comment in the last post suggests that to seek a solution to a mechanical failure, isolation is the best method. I don't see that you are seeking a solution to a failure but trying to get someone to create a Maxim, ie a chiseled in stone definition to support some preconceived notions that you might have.

    The very fact that there are hundreds of different boat profiles in existence that offer various advantages for various parameters should suggest to you that more information is required if you want relevant help

    So before you chew up another contributor,
    I have suggested that the round bow and stern profile is best for a slow moving barge, supported only by empirical date that you can find off the internet.
    I also gave such a simplified reason for your query of "WHY? " as smoother acceleration of the water at the bow and some pressure recovery at the stern.

    I have never seen a book called " Naval Architect Concepts for Dummies"

    So to reiterate, to answer the WHY part, is extremely complex to be accurate.

    How can one explain the theory of dynamics, hydrodynamics, vortices, drag coefficients,
    boundary layer theory, curls, and the list goes on and on with a simple post.

    The theme from those that are willing to help you is this " the answers are not simple, and change with various parameters, give us the parameters and we can offer some suggestions"
     
  12. Justaguy
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    Justaguy Junior Member

    Last one

    Barry,

    Thanks for the explanation, especially that part related to my specific question.

    As for what I was after, whether my original question was clear enough or not, I have since explained more and clarified a few times. I hesitated to offer any specifications at all, because I feared exactly what ended up happening -- the topic got derailed and off in a direction that I never intended. And whether a person can see this or not, in fact, the less info I provided along the way, the closer we came to actually answering my question. The more info I provided, the more derailed it became. Counter-intuitive to some, but I saw that coming in the beginning and tried to avoid it. I failed.

    As to the "chewing", I started off polite and when the disparagement came my way, I pushed back. Thereafter, I tried repeatedly to get it back on track. Take that as you will.

    For the record, I think naval architects are generally talented, educated people, and that this forum benefits significantly from their participation. On the other hand, it has been my experience that the ability to know and the ability to explain often do not reside in the same person. I believe that a PhD or engineer should be able to condense and explain what he does to a 10-year-old. If he can't, then I start to wonder if he really doesn't understand it himself. However, there are people who certainly disagree with me on that. That's OK. Opinions and viewpoints are what they are.

    Though not what I hoped for, I appreciate the information that I was able to gain via this thread, and thank those who provided it. As for the others, I'm sorry we couldn't see eye-to-eye on this one.

    In my opinion, we're beating a dead horse here, so I'm going to stop.

    Justaguy
     
    Last edited: Feb 6, 2016
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    The thread topic was about "hull shape", "what is best", not isolating the bow ! Unfortunately, scant information was given about what the "hull" was intended for, little wonder it went nowhere.
     
  14. rxcomposite
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    rxcomposite Senior Member

    This seems to be a good discussion but unfortunately without some basic knowledge, the discussion becomes muddy with the OP not knowing to ask the right question.

    It starts with a box design, plumb sides, plumb bow/stern. The block coefficient Cb or the fullness of the hull is 1.

    When the front end is sloped, the Cb becomes less, the longitudinal center of buoyancy LCB shifts and further as the hull shape (ship types) transitions. It can shift forward or aft as the illustration shows.

    As the ship types progresses from the slow barge to fast ships like containers and military type ships, there are other corresponding parameters that change or will work for the type of ship. This progression is best measured by the froude number or the taylors number, the ratio of speed to length. That is, a low froude number means a slow bulky ship, a high froude number a fast ship.

    The first three images shows a box barge, a barge with slope front, a barge with sloped front and end. There is a corresponding Cb and LCB change.

    These Cb corresponds to what is best in terms of speed to length ratio (froude number) in the 3rd illustration. Thus, very slow ship with plumb sides would not mind plowing thru the water. Sloping the bow would mean it will be better at battling the head water and thus have less resistance. Sloping the end would mean better flow of the water as it tries to converge back again and not be hampered by sudden transitions, causing residual resistance.

    Rounding the bow like a spoon is an improvement and tapering the ends would mean lesser resistance. This is a "fish shape". As it goes faster, the ends are further refined and pointed and includes the width (beam) of ship. Note that even if you have a very fine ship, the resistance would almost be the same with the box barge if towed at a very slow speed, hence the froude number matters.

    The attached sheet is based on the simplified assumptions written by various notable authors and establishments.
     

    Attached Files:


  15. philSweet
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    philSweet Senior Member

    Justaguy,

    For what ever reason, you believe that your question about shape is a good way to begin to understand barge performance. It isn't.

    You think it has an easy answer. It doesn't.

    You think the answer will be the same for most barges. It isn't.

    You don't think highly specialized math is needed to get an approximate answer or understand why. It is. It is harder than most of what NASA engineers actually do. I was a missile guy and did missile math. Now I'm retired and struggle with boat math.

    You think "how hard can it be - one moving part, steady speed, steady direction, whats the problem". The problem is that you have to calculate the path taken by millions of water droplets as they flow around the boat. You have to define the entire flow around the whole boat and far into the wake. And the pressure everywhere around the boat. And the wavy surface created by the boat. And the waves that are already on the water. Do that and we can start making some generalities. With out that, you've got zip. And we still aren't very good at calculating that stuff for arbitrary shapes. Nobody would promise you an answer within 10%, even if you paid them thousands of dollars. Ok, Yobarnacle probably could. He drove tugs for half a century.

    Boat shape is a very hard nut to crack. Until recently, it was done by building a few million boats over 6000 years and seeing what worked. Naval battles and hurricanes kept it a lively business. And finding a new continent every now and then encouraged investment. Lots of investment. Like we spend billion dollars a year now a days to answer the same question - what should it look like.

    I suggest you begin by learning boat speak. A dictionary is pinned near the top of the "all things boating" sub forum. The odds of getting an answer improve if we speak the same language. And regarding this, it is you that have to come to us since you haven't posted your boat dictionary for us to read. Think of it as meeting us half way.

    The first thing you need to do is stop thinking in 2D. There aren't any 2D best shape answers. Everything is 3D. Water flows in 3D.

    If you wanted to build a barge, you would need to comply with the building regulations just like a house. So you take courses to learn the regulations. Your barge has to fit on the body of water - more regulations. Your barge has to play well with other barges - additional requirements. It has to be loaded and unloaded without flipping over. It has to protect the product. This is where you begin. Shape in the water comes later. There are no general shape answers because in the real world, their aren't any general shape questions, only a great many very particular ones. You look for a shape which retains dozens of performance characteristics, economic, physical, and regulatory.

    If you want to study a good shape, study traditional sharpies. There were five shops side by side each building 1 per week for fifty years in New Haven. They launched on Saturday and raced on Sunday and then went clamming until they sank. Go figure out WHY they ended up like they did after 12,000 were built by the same people. The things that contribute to shape are mostly not what you think they are. Boats that always come home remain popular. Cheap boats remain popular. Fast boats remain popular. You need to learn what constrains the shape of each boat before worrying about blue sky optimization.

    Real world optimal shape solution.
    [​IMG]
     
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